<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .san{
      height: 0;
      width: 0;
      border-top: 80px solid transparent;
      border-left: 80px solid transparent;
      border-right: 80px solid transparent;
      border-bottom: 80px red solid;
    }
  </style>
</head>
<body>
  <div class="san"></div>
</body>
</html>